Zabb City

This arguably used to be one of the better Thai restaurants in Manhattan but may no longer be, though it is one of three or so in Manhattan where anyone can get very spicy food. They have a 1-5 spiciness index, and anything 4 and up will seriously challenge most people (3 is already robustly spicy). Zabb specializes in Isaan cuisine from the Northeastern part of Thailand near Laos, so get specialties like larb, yum, som tom, and toam. If you want your food less sweet and more sour, ask. You might not get it exactly the way you want, but it's worth a try.